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add reflection to readme

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Pekka Laiho 2020-12-05 09:02:53 +07:00
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@ -379,6 +379,34 @@ PI | PHP constant `M_PI`
\_\_DIR\_\_ | Directory of a file being evaluated using the special form `load`. Otherwise null. \_\_DIR\_\_ | Directory of a file being evaluated using the special form `load`. Otherwise null.
\_\_FILE\_\_ | Filename of a file being evaluated using the special form `load`. Otherwise null. \_\_FILE\_\_ | Filename of a file being evaluated using the special form `load`. Otherwise null.
## Reflection
You can use the `meta` function to retrieve the arguments, body or full code of user-defined functions:
```
> (def add (fn (a b) (+ a b)))
<function>
> (meta add "args")
(a b)
> (meta add "body")
(+ a b)
> (meta add "code")
(fn (a b) (+ a b))
```
This allows for some fun tricks. For example, we can retrieve the body of a function and evaluate it as part of another function:
```
> (def addOne (fn (n) (+ n 1)))
<function>
> (def addTwo (fn (n) (+ n 2)))
<function>
> (def addBoth (fn (n) (+ (eval (meta addOne "body")) (eval (meta addTwo "body")))))
<function>
> (addBoth 1)
5
```
## Extending ## Extending
The project is easy to extend because it is trivial to add new functions whether the implementation is defined on the PHP or Lisp side. The project is easy to extend because it is trivial to add new functions whether the implementation is defined on the PHP or Lisp side.